1. Field of the Invention
The present invention relates to vapor-mist dielectrics, and more specifically to improved electrical strength of vapor-mist dielectrics before voltage application.
2. The Prior Art
The electrical strength of vapor-mist dielectrics results from a combination of the vapor from the droplets enhancing the gas strength and the droplets collecting electrons and ions. The droplets collecting electrons and ions helps prevent the formation of electron avalanches which precede breakdown. Generally, as the mist density increases, the electron collecting properties are enhanced and more electron avalanches are extinguished, and the electrical breakdown strength is improved. Ideally the highest density of mist should be near the electrode surfaces where the electrical stress is highest.
In addition, it is well known that gases such as SF.sub.6 have increased AC breakdown strength in non-uniform fields by corona stabilization or a space charge which develops around sharp edged electrodes. In this way, a needlepoint electrode appears more as a spherical shape at the tip and the AC breakdown strength of certain non-uniform field gaps approaches that of uniform fields. This approach using fine wire electrodes in various electrode configurations has been used. See Uhlig, "The Ultra Corona Discharge, A New Discharge Phenomena Occurring on Thin Wires" National Research Counsel of Canada, 1956, pp. 15-1-15-3. The problem with using fine wire electrodes with power frequency voltages is that a large amount of electrical energy is consumed in generating the required amount of space charge and the system therefore is very inefficient. Also, this method is not adaptable to work under impulse voltage (lasting only microseconds) conditions because there is insufficient time for the necessary space charge to form. An advantage in using vapor-mist with fine wire instead of solid electrodes is better uniformity and flow of mist as it can pass through the electrode body.
There remains therefore a need for improving the electrical strength and performance of vapor-mist by ensuring that the most dense mist is at the electrode surfaces, or regions of highest electrical stress before the application of electrical voltages. Also, there is a need for utilizing fine wire electrode systems without excessive energy consumption at power frequency voltages and also for using fine wire systems with short time impulse voltages.